A conventional fixed-cutter, or “drag,” rotary drill bit for drilling subterranean formations includes a bit body having a face region thereon carrying cutting elements for cutting into an earth formation. The bit body may be secured to a hardened steel shank having a threaded pin connection, such as an API threaded pin, for attaching the drill bit to a drill string that includes tubular pipe segments coupled end to end between the drill bit and other drilling equipment. Equipment such as a rotary table or top drive may be used for rotating the tubular pipe and drill bit. Alternatively, the shank may be coupled to the drive shaft of a down hole motor to rotate the drill bit independently of, or in conjunction with, a rotary table or top drive.
Typically, the bit body of a drill bit is formed from steel or a combination of a steel blank embedded in a particle-matrix composite material that includes hard particulate material, such as tungsten carbide, infiltrated with a molten binder material such as a copper alloy. The hardened steel shank generally is secured to the bit body after the bit body has been formed. Structural features may be provided at selected locations on and in the bit body to facilitate the drilling process. Such structural features may include, for example, radially and longitudinally extending blades, cutting element pockets, ridges, lands, nozzle ports, and drilling fluid courses and passages. The cutting elements generally are secured to cutting element pockets that are machined into blades located on the face region of the bit body, e.g., the leading edges of the radially and longitudinally extending blades. These structural features, such as the cutting element pockets, may also be formed by a mold used to form the bit body when the molten binder material is infiltrated into the hard particulate material. Advantageously, a particle-matrix composite material provides a bit body of higher strength and toughness compared to steel material, but still is subject to slurry erosion and abrasive wear, particularly on lower stress surface areas of the drill bit. Therefore, it would be desirable to provide a method of manufacturing suitable for producing a bit body that includes hardfacing materials that are less prone to slurry erosion and wear.
Generally, most or all of the cutting elements of a conventional fixed-cutter rotary drill bit for drilling soft and medium formations each include a cutting surface comprising a hard, superabrasive material such as mutually bound particles of polycrystalline diamond. Such “polycrystalline diamond compact” (PDC) cutters have been employed on fixed-cutter rotary drill bits in the oil and gas well drilling industries for several decades.
FIG. 1 illustrates a conventional fixed-cutter rotary drill bit 10 generally according to the description above. The rotary drill bit 10 includes a bit body 12 that is coupled to a steel shank 14. A bore (not shown) is formed longitudinally through a portion of the drill bit 10 for communicating drilling fluid to a face 20 of the drill bit 10 via nozzles 19 during drilling operations. Cutting elements 22 (typically polycrystalline diamond compact (PDC) cutting elements) generally are bonded to the face 20 of the bit body 12 by methods such as brazing, adhesive bonding, or mechanical affixation.
A drill bit 10 may be used numerous times to perform successive drilling operations during which the surfaces of the bit body 12 and cutting elements 22 may be subjected to extreme forces and stresses as the cutting elements 22 of the drill bit 10 shear away the underlying earth formation. These extreme forces and stresses cause the cutting elements 22 and the surfaces of the bit body 12 to wear. Eventually, the surfaces of the bit body 12 may wear to an extent at which the drill bit 10 is no longer suitable for use. Therefore, there is a need in the art for enhancing the wear-resistance of the surfaces of the bit body 12. Also, the cutting elements 22 may wear to an extent at which they are no longer suitable for use.
FIG. 2 is an enlarged view of a PDC cutting element 22 like those shown in FIG. 1 secured to the bit body 12. Typically, the cutting elements 22 are fabricated separately from the bit body 12 and secured within pockets 21 formed in the outer, or exterior, surface of the bit body 12 with a bonding material 24 such as an adhesive or, more typically, a braze alloy as previously discussed herein. Furthermore, if the cutting element 22 is a PDC cutter, the cutting element 22 may include a polycrystalline diamond compact table 28 secured to a cutting element body or substrate 23, which may be unitary or comprise two components bonded together.
Conventional bonding material 24 is much less resistant to wear than are other portions and surfaces of the drill bit 10 and of cutting elements 22. During use, small vugs, voids and other defects may be formed in exposed surfaces of the bonding material 24 due to wear. Solids-laden drilling fluids and formation debris generated during the drilling process may further erode, abrade and enlarge the small vugs and voids in the bonding material 24 even though partially shielded from the higher stresses caused by formation cutting. The entire cutting element 22 may separate from the drill bit body 12 during a drilling operation if enough bonding material 24 is removed. Loss of a cutting element 22 during a drilling operation can lead to rapid wear of other cutting elements and catastrophic failure of the entire drill bit 10. Therefore, there is also a need in the art for an effective method for enhancing the wear-resistance of the bonding material to help prevent the loss of cutting elements during drilling operations.
Ideally, the materials of a rotary drill bit must be extremely hard to withstand abrasion and erosion attendant to drilling earth formations without excessive wear. Due to the extreme forces and stresses to which drill bits are subjected during drilling operations, the materials of an ideal drill bit must simultaneously exhibit high fracture toughness. In practicality, however, materials that exhibit extremely high hardness tend to be relatively brittle and do not exhibit high fracture toughness, while materials exhibiting high fracture toughness tend to be relatively soft and do not exhibit high hardness. As a result, a compromise must be made between hardness and fracture toughness when selecting materials for use in drill bits.
In an effort to simultaneously improve both the hardness and fracture toughness of rotary drill bits, composite materials have been applied to the surfaces of drill bits that are subjected to extreme wear. These composite or hard particle materials are often referred to as “hardfacing” materials and typically include at least one phase that exhibits relatively high hardness and another phase that exhibits relatively high fracture toughness.
FIG. 3 is a representation of a photomicrograph of a polished and etched surface of a conventional hardfacing material applied upon the particulate-matrix composite material, as mentioned above, of a bit body. The hardfacing material includes tungsten carbide particles 40 substantially randomly dispersed throughout an iron-based matrix of matrix material 46. The tungsten carbide particles 40 exhibit relatively high hardness, while the matrix material 46 exhibits relatively high fracture toughness.
Tungsten carbide particles 40 used in hardfacing materials may comprise one or more of cast tungsten carbide particles, sintered tungsten carbide particles, and macrocrystalline tungsten carbide particles. The tungsten carbide system includes two stoichiometric compounds, WC and W2C, with a continuous range of mixtures therebetween. Cast tungsten carbide generally includes a eutectic mixture of the WC and W2C compounds. Sintered tungsten carbide particles include relatively smaller particles of WC bonded together by a matrix material. Cobalt and cobalt alloys are often used as matrix materials in sintered tungsten carbide particles. Sintered tungsten carbide particles can be formed by mixing together a first powder that includes the relatively smaller tungsten carbide particles and a second powder that includes cobalt particles. The powder mixture is formed in a “green” state. The green powder mixture then is sintered at a temperature near the melting temperature of the cobalt particles to form a matrix of cobalt material surrounding the tungsten carbide particles to form particles of sintered tungsten carbide. Finally, macrocrystalline tungsten carbide particles generally consist of single crystals of WC.
Various techniques known in the art may be used to apply a hardfacing material such as that represented in FIG. 3 to a surface of a drill bit. A welding rod may be configured as a hollow, cylindrical tube formed from the matrix material of the hardfacing material that is filled with tungsten carbide particles. At least one end of the hollow, cylindrical tube may be sealed. The sealed end of the tube then may be melted or welded onto the desired surface on the drill bit. As the tube melts, the tungsten carbide particles within the hollow, cylindrical tube mix with and are suspended in the molten matrix material as it is deposited onto the drill bit. An alternative technique involves forming a cast rod of the hardfacing material and using either an arc or a torch to apply or weld hardfacing material disposed at an end of the rod to the desired surface on the drill bit. One method of applying the hardfacing material by torch is to use what is known as oxy fuel gas welding. Oxy fuel gas welding is a group of welding processes which produces coalescence by heating materials with an oxy fuel gas flame or flames with or without the application of pressure to apply the hardfacing material. One so-called “oxy fuel gas welding” is known as oxygen-acetylene welding (OAW), which is a well accepted method for applying a hardfacing material to a surface of a drill bit.
Arc welding techniques also may be used to apply a hardfacing material to a surface of a drill bit. For example, a plasma transferred arc may be established between an electrode and a region on a surface of a drill bit on which it is desired to apply a hardfacing material. A powder mixture including both particles of tungsten carbide and particles of matrix material then may be directed through or proximate the plasma-transferred arc onto the region of the surface of the drill bit. The heat generated by the arc melts at least the particles of matrix material to form a weld pool on the surface of the drill bit, which subsequently solidifies to form the hardfacing material layer on the surface of the drill bit.
When a hardfacing material is applied to a surface of a drill bit, relatively high temperatures are used to melt at least the matrix material. At these relatively high temperatures, dissolution may occur between the tungsten carbide particles and the matrix material. In other words, after applying the hardfacing material, at least some atoms originally contained in a tungsten carbide particle (tungsten and carbon, for example) may be found in the matrix material surrounding the tungsten carbide particle. In addition, at least some atoms originally contained in the matrix material (iron, for example) may be found in the tungsten carbide particles. FIG. 4 is an enlarged view of a tungsten carbide particle 40 shown in FIG. 3. At least some atoms originally contained in the tungsten carbide particle 40 (tungsten and carbon, for example) may be found in a region 47 of the matrix material 46 immediately surrounding the tungsten carbide particle 40. The region 47 roughly includes the region of the matrix material 46 enclosed within the phantom line 48. In addition, at least some atoms originally contained in the matrix material 46 (iron, for example) may be found in a peripheral or outer region 41 of the tungsten carbide particle 40. The outer region 41 roughly includes the region of the tungsten carbide particle 40 outside the phantom line 42.
Dissolution between the tungsten carbide particle 40 and the matrix material 46 may embrittle the matrix material 46 in the region 47 surrounding the tungsten carbide particle 40 and reduce the hardness of the tungsten carbide particle 40 in the outer region 41 thereof, reducing the overall effectiveness of the hardfacing material. Dissolution is the process of dissolving a solid, such as the tungsten carbide particle 40, into a liquid, such as the matrix material 46, particularly when at elevated temperatures and when the matrix material 46 is in its liquid phase, which transforms the material composition of the matrix material. In one aspect, dissolution is the process where a solid substance enters (generally at elevated temperatures) a molten matrix material that changes the composition of the matrix material. Dissolution occurs more rapidly as the temperature of the matrix material 46 approaches the melting temperature of tungsten carbide particle 40. For example, an iron-based matrix material will have greater dissolution of the tungsten carbide particles 40 than a nickel-based matrix material will, because of the higher temperatures required in order to bring the iron-based matrix material into a molten state during application. With a change in the composition of the matrix material, the material also becomes more sensitive to slurry erosion and wear, particularly on lower stress surface areas of the drill bit and bit body. Therefore, there is a need in the art for abrasive wear-resistant hardfacing materials that include a matrix material that allows for dissolution between tungsten carbide particles and the matrix material to be minimized. There is also a need in the art for methods of applying such abrasive wear-resistant hardfacing materials to surfaces of particle-matrix composite drill bits, and for drill bits and drilling tools that include such particle-matrix composite materials.